A prior art spark plug referred to as a “bridge electrode type” spark plug is known and has the form of a bridge that extends diagonally over the front end of the shell of the spark plug. The center electrode is tipped with an end piece made of a precious metal. Opposite the center electrode, a counterpart made of precious metal is welded onto the bridge that serves as the ground electrode. The end piece and the counterpart delimit the spark gap.
The prior art spark plug is especially suitable for stationary gas engines and is characterized by a stable electrode arrangement that is suitable for a long service life.
A “bridge electrode type” spark plug, but without the precious metal tip on the center electrode and on the bridge, is known from EP Patent No. 0 134 355 A1.
The advantages of this spark plug type for use in gas engines, especially in stationary gas engines, are only fully evident if the arrangement of the center electrode and the electrode bridge can be produced with dimensional accuracy relative to one another. The cause of dimensional variations may reside in the fusing of the center electrode into the insulator, in the shrink-fitting of the insulator in the spark plug shell, in the process of crimping the back end of the spark plug shell, in the welding of the bridge onto the front edge of the spark plug shell, and in the welding of the precious metal pieces onto the bridge and onto the center electrode, and bring about dimensional variations in the spark gap, deviations from parallelism of the surfaces bordering the spark gap, and deviations in the alignment of the center electrode and the precious metal counterpart on the ground electrode coaxial to the spark plug center line. Keeping these error sources as small as possible requires great manufacturing effort and is in part responsible for a high price of the spark plugs for gas engines.